Selective CO2 Conversion with Novel Copper Catalyst

ABSTRACT

The present disclosure provides hierarchical CuO-derived inverse opal (CuO—IO) electrocatalyst compositions, their synthesis, and their application to selectively convert CO 2  into carbon monoxide (CO). The electrocatalyst compositions have a three-dimensional interconnected CuO backbone in hexagonal arrangement. In one embodiment, the compositions have an inverse structure of poly (methyl methacrylate) (PMMA) latex opal. In one embodiment, the electrocatalyst composition inverse-opal structure is comprised of copper-oxide nanoparticles having an average mean diameter ranging from about 15 to about 20 nm. In another embodiment, the compositions have an average cavity size of 175 to 185 nm.

RELATION TO OTHER APPLICATIONS

This application claims priority benefit as a continuation of U.S.Non-Provisional patent application Ser. No. 16/835,381 filed Mar. 31,2020, currently pending, which in turn claimed priority benefit as aNon-Provisional of U.S. Provisional Patent Application Ser. No.62/829,446 filed Apr. 4, 2019, currently expired, both of which areincorporated by reference in their entirety herein.

GOVERNMENT INTEREST

The United States Government has rights in this invention pursuant tothe employer-employee relationship of the Government to the inventors asU.S. Department of Energy employees and site-support contractors at theNational Energy Technology Laboratory.

FIELD OF THE INVENTION

One or more embodiments consistent with the present disclosure relate tocopper-oxide electrocatalyst compositions for conversion of CO₂ andwater to CO and H₂ (syngas). Accordingly, the disclosure includesmaterials, methods of their preparation, and methods for using thecompositions.

BACKGROUND OF THE INVENTION

Electrochemical CO₂ reduction (EC-CO₂RR) is a promising approach toconvert CO₂ emissions into industrially-relevant and value-addedchemicals and fuels. However, due to slow kinetics and multi-electrontransfer pathway, EC-CO₂RR usually requires significant overpotentialsand can suffer from poor product selectivity and competitive hydrogenevolution reaction (HER). The development of highly active, selectiveand robust CO₂ conversion catalysts is of vital interest to overcomethese drawbacks. The activity and selectivity towards specific productsstrongly depend on electrocatalyst morphology, surface roughness, natureof electrochemically active sites, electronic configuration, transportlimitations, local pH environment at the electrode surface, etc.

To date, numerous materials have been widely studied, including metals,oxides, and carbonaceous composites. Expensive metals such as gold andsilver can selectively convert CO₂ into CO, a commodity chemical used ina variety of industrial processes, including methanol andFischer-Tropsch synthesis, among others. Copper derived materials havealso attracted much attention due to their low cost, high abundance, andability to produce hydrocarbons or oxygenated hydrocarbons, and effortshave recently focused on structural control to improve their productselectivity. A number of different structures and dimensions ofcopper-based catalysts have been investigated, such as nanoparticles,nanofoams, nanowires, prisms, dendrites, etc. CuO-derived hierarchicalnanostructures composed of nanowires exhibited selective CO and HCOOHproduction with a total FE of 82.5% at −0.55 V vs. RHE that wasattributed to the 3D porous structure of catalysts. MesoporousCu₂O-derived foams were also found to selectively produce C₂H₄ and C₂H₆with a maximum C₂ FE reaching 55% at −0.9 V vs. RHE owing to thepresence of dominant (100) surface sites for C—C coupling and temporaltrapping of gaseous intermediates inside the mesopores. Despite thisprogress, it is still challenging to fully understand the nature ofelectrochemically active sites because the product selectivity of coppercatalysts strongly depends on their structure, morphology, and oxidationstate.

Inverse opal (TO) materials have been widely studied for applications incatalysis, photonics, photovoltaic devices, energy conversion, andenergy storage. The three-dimensional (3D) interconnected, highly porousstructure of IOs are arranged in hexagonal close packed framework andoffer large surface-to-volume ratio and better adsorbability of reactantmolecules. Despite these benefits, few studies on IO catalysts forEC-CO₂RR have been reported. Porous mesostructured Au and Ag IOcatalysts have shown improved CO selectivity due to the generation of pHgradients that reduced proton availability at the catalyst surface andsuppressed competitive H₂ evolution. Zhang and coworkers found improvedCO selectivity (˜45%) for cube-like Cu—IO, but the oxidation state andcrystallographic orientation during EC-CO₂RR were not investigated.However, larger IO pore size significantly decreases CO FE whileenhancing H₂ and C₂ formation.

Accordingly, it is an object of this disclosure to provide copper-oxide(Cu—O) based electrocatalysts for the reduction of CO₂ to CO. The Cu—Ocatalysts have a 3D inverse opal structure and demonstrate high Faradaicefficiencies and current densities, high electrostability, and do notrely on precious metals. It is also an object of this disclosure toprovide methods for making the compositions, as well as for use of thecompositions. These and other objects, aspects, and advantages of thepresent disclosure will become better understood with reference to theaccompanying description and claims.

SUMMARY OF THE INVENTION

The present disclosure provides hierarchical CuO-derived inverse opal(CuO—IO) electrocatalyst compositions, their synthesis, and theirapplication to selectively convert CO₂ into carbon monoxide (CO). Theelectrocatalyst compositions have a three-dimensional interconnected CuObackbone in hexagonal arrangement. In one embodiment, the compositionshave an inverse structure of poly (methyl methacrylate) (PMMA) latexopal. In one embodiment, the electrocatalyst composition inverse-opalstructure is comprised of copper-oxide nanoparticles having an averagemean diameter ranging from about 15 to about 20 nm. In anotherembodiment, the compositions have an average cavity size of 175 to 185nm.

The catalyst compositions provide a 3˜10-fold enhancement in productselectivity with improved CO₂ conversion rates and reaction efficiencycompared to currently commercially available materials and similarmaterials in the open scientific literature. The improvement incatalytic rates, efficiencies, selectivity, and overpotential biasesaddress core technical issues that have prevented the development ofeffective electrocatalytic technologies for CO₂ utilization.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the multipleembodiments of the present invention will become better understood withreference to the following description, appended claims, and accompanieddrawings where:

FIG. 1 depicts (A) SEM image, (B) HR-TEM micrograph, (C) SXRD patternand (D) XANES Cu K-edge of as prepared CuO—IO, while spectra in (D) arebulk CuO, bulk Cu₂O and Cu foil standards.

FIG. 2 depicts photographs of PMMA film on (A) ITO substrate, (B)top-view and (C) cross-sectional SEM images of PMMA dry opal film.

FIG. 3 depicts (A, B) top-view and (C) cross-sectional SEM images ofas-prepared CuO-catalyst layer on hydrophilized glass substrate.

FIG. 4 depicts Fourier-transformed R-space Cu K-edge EXAFS spectra (notphase corrected) of as-prepared CuO—IO, bulk CuO, bulk Cu₂O and Cu foilstandards.

FIG. 5 depicts (A) XPS survey, (B) Cu 2p core-level, and (C) Auger CuLMM, and (D) s-XAS Cu L-edge (in TEY and PFY modes) spectra ofas-prepared CuO—IO.

FIG. 6 depicts (A) Potential-dependent Faradaic efficiencies for CO₂reduction products over CuO—IO catalyst in CO₂ saturated 0.1 M KHCO₃.(B) Long-term electrocatalytic performance of CuO—IO catalyst at −0.6 Vvs. RHE. Comparisons of (C) CO Faradaic efficiency (FE) and (D) COpartial current density (j_(CO)) at various negative potentials forCuO—IO, ˜50 nm CuO NPs, and bulk CuO powder.

FIG. 7 depicts chronoamperometric profiles showing total current densityas function of applied constant potential for CuO—IO catalyst (loadingof 2.8 mg cm_(geo) ⁻²).

FIG. 8 depicts selectivity of CO₂ reduction products for CuO—IO catalystat different applied potentials.

FIG. 9 depicts total Faradaic efficiency for C₁ products at variousapplied potentials of CuO—IO catalyst.

FIG. 10 depicts SEM images of CuO—IO/carbon paper working electrodeswith CuO—IO loading of (A, B) 1.5 mg cm_(geo) ⁻² and (D, E) 15 mgcm_(geo) ⁻²; and (C, F) corresponding CO FE vs. applied potentials.

FIG. 11 depicts steady-state current density and FE for CO production at−0.6 V vs. RHE as using graphite counter electrode (in CO₂-saturatedKHCO₃, CO₂ flow rate of 20 mL min⁻¹, catalyst loading of 2.8 mg cm_(geo)⁻²).

FIG. 12 depicts Faradaic efficiency for CO, CH₄ and H₂ at differentcathodic potentials over bare carbon paper (in CO₂-saturated KHCO₃electrolyte, CO₂ flow rate of 20 mL min⁻¹).

FIG. 13 depicts (A) XPS survey, (B) Cu 3p-Pt 4f, (C) Cu 3s-Pb 4f, (D) Zn2p, and (E) Fe 2p core-level spectra of CuO—IO post-electrodes afterlong-term run at −0.6 V vs. RHE using Pt wire and graphite counterelectrodes.

FIG. 14 depicts (A-D) SEM images of CuO—IO electrode at different spotsafter 24-hour electrolysis at −0.6 V vs. RHE, (E) EDX analysis.

FIG. 15 depicts (A, B) SEM images, (C) XRD patterns, and (D) XAS CuL-edge of CuO NPs and bulk CuO.

FIG. 16 depicts Faradaic efficiency for EC-CO₂RR products as a functionof potentials for ˜50 nm diameter CuO NPs.

FIG. 17 depicts Faradaic efficiency for EC-CO₂RR products as a functionof potentials for bulk CuO powder.

FIG. 18 depicts CO selectivity at various negative potentials forCuO—IO, ˜50 nm diameter CuO NPs, and bulk CuO powder.

FIG. 19 depicts (A) Total current density vs. cathodic potential and (B)Tafel plot for CO production over CuO—IO, ˜50 nm CuO NPs, and bulk CuOpowder.

FIG. 20 depicts (A) Cyclic voltammetry of fresh CuO—IO electrode in CO₂saturated 0.1 M KHCO₃. (B) Potential-dependent k²-weighted R-space EXAFSanalysis (no phase correction) from −0.2 V to −1.2 V vs. RHE (collectedat 30 min at each potential). (C) In situ Raman spectra for trackingsurface structure of CuO—IO during EC-CO₂RR at −0.6 V (using 785 nmlaser source). (D) SXRD patterns of CuO—IO electrode under open circuitand steady state at −0.6 V vs. RHE (*indicates residual carbon paperfeatures and Kapton window from background subtraction).

FIG. 21 depicts in situ Cu K-edge XANES of CuO—IO catalyst duringchronoamperometry from −0.2 V to −1.2 V vs. RHE (collected at 30 min ateach potential, with comparison to Cu foil standard).

FIG. 22 depicts in situ (A) Cu K-edge XANES and (B) EXAFS measurementsfor real time tracking the oxidation state changes of CuO—IO duringEC-CO₂RR condition at −0.6 V. The results at steady state are very closeto prominent features in Cu foil standard, indicating the reduction ofCuO towards metallic copper under working conditions.

FIG. 23 depicts ex situ Raman spectra of CuO—IO/carbon paper electrodesbefore (top) and after 24-h durability test at −0.6 V vs. RHE (bottom)using 633 nm laser.

FIG. 24 depicts in situ Raman spectra of CuO—IO during EC-CO₂RR atvarious applied potentials (using 785 nm laser source).

FIG. 25 depicts Raman analysis of CuO—IO collected after CO₂electrolysis at −0.6 V vs. RHE and returned to open circuit (785 nmlaser source).

FIG. 26 depicts (A) In situ time-resolved SXRD data for tracking thecrystallographic changes of CuO—IO from CuO phase under open circuit tometallic Cu during EC-CO₂RR at −0.6 V vs. RHE (* indicates residualcarbon paper features and Kapton window from background subtraction).(B) Relative peak intensity ratio of Cu (111) and Cu (200) andcrystallite size of Cu as a function of electrolysis time showing theconsistency over five hours at −0.6 V vs. RHE.

FIG. 27 depicts SXRD patterns of CuO—IO electrode collected after −0.6 Vreduction for 5 hours and returned to open circuit for 120 minindicating the gradual re-oxidation of copper to oxide phases over time(* indicates residual carbon paper features from backgroundsubtraction).

FIG. 28 depicts non-Faradic double layer charging current plottedagainst scan rate for double layer capacitance estimation for (A)CuO—IO, (B) ˜50 nm diameter CuO NPs, and (C) bulk CuO electrodes (inCO₂-saturated 0.1 M KHCO₃ electrolyte).

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modecontemplated by the inventor for carrying out the invention. Variousmodifications, however, will remain readily apparent to those skilled inthe art, since the principles of the present invention are definedherein specifically to provide description of hierarchical oxide-derivedcopper inverse opal (CuO—IO) electrocatalyst compositions, methods oftheir preparation, and methods for using such compositions.

The hierarchical oxide-derived copper inverse opal (CuO—IO)electrocatalysts provide selective CO production and strong suppressionof H₂ evolution over a wide potential window compared with typicaloxidized copper materials. The CuO—IO compositions are characterized bytheir structure, high Faradaic efficiencies, and high electrostability.

The compositions are in general terms synthesized through a three-partprocess. First, an opal template is supplied. Second, the catalystmaterial is supplied to the opal template such that the catalystmaterial infiltrates the structure of the opal template. Third, thecatalyst structure is fixed, and the template removed. In a non-limitingexample of the general method, first, an opal template is formed byevaporation-induced vertical deposition of PMMA latex. Second, the latexis infiltrated by Cu catalyst material. Third, the infiltrated latex isannealed in air to remove the opal template and form the Cu oxideinverse opal.

As noted above, inverse opal (TO) materials provide a three-dimensional(3D) interconnected, highly porous structure arranged in hexagonalclose-packed framework. IO materials offer large surface-to-volume ratioand better adsorbability of reactant molecules. As noted, such IOmaterials are formed by the infiltration of the precursor IO materialinto a suitable template substrate, followed by formation of the IO andremoval of the substrate. Suitable template materials include poly(methyl methacrylate) spheres, polystyrene spheres, carboxylicpolystyrene spheres, poly(styrene-methyl methacrylate-3-sulfopropylmethacrylate, potassium salt) spheres, poly(n-butyl acrylate-acrylicacid) spheres, carbon spheres, silica spheres, or other suitablespherical templates. Accordingly, such materials will provide a templatefor the hexagonal framework of the inverse opal structure, as determinedthrough electron microscopy, X-ray diffraction, UV-Vis-NIR absorptionspectroscopy, X-ray photoelectron spectroscopy, and X-ray absorptionspectroscopy.

The templates may be contacted with the Cu catalyst precursors using oneor more of methods in order to facilitate the formation of the IOstructure. Such methods include infiltration in ambient environment orunder vacuum, chemical vapor deposition, chemical bath deposition,electrochemical deposition, atomic layer deposition, convectiveself-assembly, evaporative co-assembly, nanoparticle suspension, orother suitable techniques. In carrying infiltration as used in theexamples below, the template is contacted with a solution including thecatalyst precursor such that the catalyst precursor will infiltrate andfill the interstitial voids of the template. The solvent is then allowedto evaporate such that the catalyst precursor remains fixed within thetemplate.

Following infiltration, the Cu catalyst precursor is structurallystabilized and the opal template may be removed. One such method tocarry out the stabilization and removal is by thermal annealing. Inannealing, the infiltrated template is heated to a temperaturesufficient to fix the structure of the Cu catalyst precursor to that ofthe CuO catalyst and the opal template is removed by decomposition. Theannealing operation thus results in catalyst retaining the final inverseopal structure. Other methods of fixing the catalyst precursor mayinclude wet chemical etching, plasma treatment or Ozone oxidation.

The resulting hierarchical oxide-derived copper inverse opal (CuO—IO)electrocatalyst compositions have structure characterized by the inverseof the supporting template, that is, they have the structure of theinterstitial voids of the template. The compositions thus have ahexagonal structure comprised of CuO nanoparticles. In one embodiment,the CuO nanoparticles have a mean average diameter of 15-20 nm.

The electrocatalyst compositions are of use in the conversion of CO₂ andH₂O to CO and H₂ (syngas). One exemplary method utilizing theelectrocatalyst compositions includes loading the catalyst onto asuitable electrode material, placing the catalyst containing electrodeinto an aqueous electrolyte containing dissolved CO₂, and applying anegative potential to the electrocatalyst containing electrode. Anotherexemplary method includes constructing a gas diffusion-style electrodeor membrane electrode assembly using the catalyst. This type ofconfiguration would be used to assembly an electrolyzer-style electrodewhere negative electrode potentials are applied to convert humidifiedCO₂ gas streams into CO.

The electrocatalyst compositions of the present disclosure provide highselectivity in the production of CO. The CO selectivity is defined asthe ratio between the production rate for CO and total production ratefor all reduced products, including CO₂-derived products and H₂ evolvedfrom water splitting. In one embodiment, the electrocatalystcompositions provide a CO selectivity greater than about 80%. In anotherembodiment, the electrocatalyst compositions provide a CO selectivitygreater than about 90%.

${FE_{i}} = \frac{z_{i}*F*n_{i}}{I*t}$

The electrocatalyst compositions of the present disclosure also providehigh CO Faradaic efficiency. Faradaic efficiency (FE) is defined as thepercentage of supplied electrons used to convert CO₂ into products suchas CO, and is calculated by dividing the quantity of produced productmolecules by the number of supplied electrons compared with thetheoretical number of electrons required to form that quantity ofproduct molecules. Specifically:

where

_(i) is the number of electrons involved in the formation of i product (

=2 for CO, H₂, and HCOOH,

=8 for CH₄,

=12 for C₂H₄, and z=14 for C₂H₆); F is the Faraday's constant; n_(i) isthe number of moles of product i formed (determined by GC and IC); I isthe total current; and t is electrolysis time. In one embodiment, theelectrocatalyst compositions provide a Faradaic efficiency of greaterthan about 65%. In another embodiment, the electrocatalyst compositionsprovide a Faradaic efficiency of greater than about 70%.

In one embodiment, the electrocatalytic composition comprisescopper-oxide having an inverse-hexagonal opal structure; where theinverse-opal structure is a negative replica of poly (methylmethacrylate) opal; where the electrocatalyst has an average cavity sizeranging from about 175 to about 185 nm; where the composition has aFaradaic efficiency greater than about 70% at −0.6 V vs. RHE; and wherethe composition has a CO to H₂ selectivity up to about 90% at −0.7 V vs.RHE.

Examples Synthesis

A poly (methyl methacrylate) (PMMA) latex was prepared bysurfactant-free emulsion polymerization using a cationic free radicalinitiator. Deionized water (DIW) (875 mL) and methyl methacrylate (100g) were mixed at room temperature under a nitrogen flow for 30 min andthen maintained at 70° C. Subsequently, a solution containing 0.15 g of2,2′-azobis (2-methylpropionamidine) dihydrochloride and 25 mL of DIWwas quickly added. A milky white suspension was formed, and thesuspension was maintained at 70° C. for 6 h to complete thepolymerization. After cooling to room temperature for 1 h, theconcentration of obtained PMMA latex (diameter of ca. 210 nm) was 10 wt%.

Bare glass substrates were cut into 1 cm×3 cm pieces and cleaned with amixture of acetone, isopropanol and deionized water (DIW) for two hoursand then immersed in aqueous sodium hydroxide solution (0.5 M) for atleast six hours to hydrophilize the surface. The hydrophilizedsubstrates were finally rinsed by DIW and dried under N₂ flow.

PMMA opal films were grown by the evaporation-induced verticaldeposition technique. The stock PMMA colloidal suspension was diluted inDIW to achieve the concentration of 0.5 wt %. The hydrophilizedsubstrate was partially immersed into 5 mL of PMMA solution with anangle of 45˜60° and left in an electric oven at 35° C. with controlledhumidity of ˜80% for several days to form a self-assembled opal in a fcccrystalline lattice over an area of 1 cm×1.5 cm. The opal film was thensintered at 80° C. for 30 minutes to enhance the domain arrangement andmechanical stability.

The CuO—IO electrocatalyst compositions were then prepared byinfiltration of copper precursor solution with the PMMA opal film. 20 μLof copper precursor solution including Cu(NO₃)₂·3H₂O (0.625 g),C₆H₈O₇·H₂O (0.375 g), and C₂H₅OH (10 mL, 200 proof) was penetratedslowly into 10°-tilted PMMA opal and naturally evaporated overnight. Theinfiltrated film was subsequently annealed in air at 400° C. withramping rate of 1° C. min⁻¹ for 4 h to completely remove PMMA andreassemble hierarchical CuO inverse opal (namely CuO—IO) as a negativereplica of bare PMMA opal.

Electrochemical CO₂ Reduction Measurement

Electrochemical CO₂ reduction experiments were carried out in agas-tight, two-compartment H-cell separated by a Nafion 117 protonexchange membrane. Each compartment was filled with 50 mL of aqueous 0.1M KHCO₃ electrolyte (99.99%, Sigma-Aldrich) and contained 100 mLheadspace. The catholyte was continuously purged with CO₂ (99.999%,Butler gas) at a flow rate of 20 mL min⁻¹ (pH˜6.8) during theexperiments and stirred at 200 rpm. The counter and reference electrodeswere Pt wire and Ag/AgCl (saturated NaCl, BASK)), respectively. Thecatalyst ink was prepared by dispersing 4 mg of as-prepared CuO—IO(scraped down from the glass substrates) in 200 μL of methanol and 10 μLof Nafion® 117 solution binder (Sigma-Aldrich, 5%). Working electrodeswere fabricated by drop-casting the prepared ink onto PTFE-coated carbonpaper gas diffusion layer (Toray paper 060, Alfa Aesar). The as preparedCuO—IO loading on carbon paper was kept at 2.8±0.1 mg cm_(geo) ⁻² (basedon geometric area) unless otherwise noted.

CO₂ reduction examples were performed at ambient temperature andpressure using a SP-300 potentiostat (BioLogic Science Instrument). Allpotentials were referenced against the reversible hydrogen electrode(RHE) and the uncompensated resistance was automatically corrected at85% (iR-correction) using the instrument software. Typical workingelectrode resistances were 30-40Ω. Short-term chronoamperometricexamples were conducted for 30 min at each applied potentialsequentially between −0.2 V and −1.2 V vs. RHE. Long-termchronoamperometric examples were conducted for 24 hours at −0.6 V vs.RHE. The total and partial current densities were normalized to theexposed geometric area. Each data point is an average of at least threeindependent experiments on different fresh electrodes. The evolved gasproducts were quantified by PerkinElmer Clarus 600GC equipped with bothFID and TCD detectors, using ShinCarbon ST 80/100 Column and He as acarrier gas. The GC was calibrated regularly using a calibration mixtureof gases with known composition. The liquid products in the catholyteswere determined by Dionex ICS-5000+ ion chromatography using ED50conductometric detector, ASRS suppressor in auto-generation mode,AS11-HC column and KOH eluent with a gradient of 0.4-30 mM in 45 minrun.

Materials Characterizations

The electron microscopy images in FIG. 1A and FIGS. 2-3 show athree-dimensional interconnected CuO backbone with an average cavitysize of 180±5 nm. The HR-TEM micrograph in FIG. 1B reveals the CuO—IOstructure is composed of 15˜20 nm CuO nanoparticles with latticespacings of 0.272 nm, 0.252 nm, and 0.232 nm that can be indexed to(110), (002), and (200) planes of polycrystalline CuO, respectively.

The synchrotron XRD pattern of CuO—IO in FIG. 1C displays severaldiffraction peaks representative of monoclinic CuO (space group C₂/c)with lattice constants a=4.7119 Å, b=3.4350 Å, c=5.1164 Å. Neithercuprite Cu₂O nor metallic copper phases were found, and the mean CuOcrystallite size of ca. 15 nm closely matched particle sizes determinedfrom HR-TEM imaging. Cu K-edge XANES spectra (FIG. 1D) reveal that theline shape, and the positions of pre-edge (1s→3d transition), shakedownfeature (1s 4p transition) and white line for CuO-TO resemble the CuOstandard, indicating Cu²⁺ oxidation state. Two maxima centered at 1.53and 2.52 Å in corresponding Fourier transformed k²-weighted EXAFSspectrum (FIG. 4 ) are ascribed to Cu—O bond in the nearest neighborshell and Cu—Cu in the next near neighbor coordination, respectively.Additional XPS, Auger and Cu L-edge XAS data in FIG. 5 also confirmedthe presence of CuO.

Performance

The CO₂ electroreduction activity of the CuO—IO electrocatalystcomposition was evaluated using chronoamperometry between −0.2V and−1.2V vs. the reversible hydrogen electrode (vs. RHE) as shown in FIG.6A. High C₁ selectivity was demonstrated over an extremely widepotential range, with only minor C₂H₄ and H₂ between −0.9 V and −1.2 Vand trace ethane −1.2 V (<0.1% FE) (FIGS. 8-9 and Table 1). Totalgeometric current densities of CuO—IO were comparable to otheroxide-derived copper electrocalysts (FIG. 7 ); however, C₁ productselectivity was much higher than expected for copper-based catalysts.Methane and formic acid were the dominant products below −0.4 V, whileCO production increased to a maximum Faradaic efficiency (FE) of 72.5%at −0.6 V and maximum CO selectivity up to nearly 90% was observedbetween −0.7 V and −0.8 V (FIG. 8 ). An average C₁ FE of 78±2% wasobserved between −0.2 to −1.1V vs. RHE (FIG. 9 ), which decreased toapproximately 60% at −1.2V vs. RHE due to the increased HER at largeoverpotentials. The CO yield increased from 30 μmol at −0.2 V to 60˜70mmol of CO per gram of catalyst per hour at potentials more negativethan −1.0 V (Table 1). Control experiments with different CuO—IOcatalyst loadings, varied catalyst layer thickness, and a graphitecounter electrode resulted in similarly high CO selectivity (FIGS. 10-11). Finally, the bare carbon paper demonstrated almost exclusive H₂production with only trace CO and CH₄ detected from −0.7 V to −1.2 V(FIG. 12 ).

Long-term CO₂ electrolysis demonstrated consistent CO selectivity forthe CuO—IO electrocatalyst. As shown in FIG. 6B, an average 67±2% CO FEwas found over 24 hours at −0.6 V with a stable current density of ca.2.5 mA cm⁻² and no detectable H₂ evolution. XPS analysis ofpost-reaction electrodes after long-term runs at −0.6 V using Pt wireand graphite counter electrodes ruled out significant deposition oftrace Pt, Zn, Pb or Fe elements onto the electrode surface (FIG. 13 ).Post reaction electron microscopy in FIG. 14 revealed the catalystpreserved its general inverse opal structure. SEM images of CuO—IOelectrode at different spots after 24-hour electrolysis at −0.6 V vs.RHE demonstrated that CuO—IO retained its structure after durabilitytest despite partial structural dissolution into nanoparticleagglomerates (occupying approximately 20-30% of entire post-electrode).Post-reaction EDX did not identify Pt. The sustained CO selectivity andcurrent density over 24-hour operation indicates the CuO—IO catalyst isa robust CO₂-to-CO conversion catalyst.

TABLE 1 Potential dependent Faradaic efficiencies (FE, %) and formationrate (r, mmol g_(catalyst) ⁻¹ h⁻¹) of EC-CO₂RR products for CuO-IO. Vvs. CO CH₄ C₂H₄ C₂H₆ HCOOH H₂ RHE FE r FE r FE r FE r FE r FE r −1.231.1 64.2 6.7 0.7 1.9 0.7 <0.1 6.6 × 10⁻³ 25.6 39.7 20.4 50.1 −1.1 36.159.2 17.6 7.2 2.8 0.78 — — 23.3 22.9 10.5 17.3 −1.0 54.9 68.0 16.0 5.33.6 0.9 — — 11.4 11.3 4.3 6.1 −0.9 49.1 43.5 7.2 1.5 0.45 5.8 × 10⁻² — —14.7 11.7 5.7 4.5 −0.8 63.2 28.2 2.7 0.3 — — — — 7.8 3.9 — — −0.7 66.516.1 1.8 0.1 — — — — 5.5 1.9 — — −0.6 72.5 8.6 1.0 2.9 × 10⁻² — — — —9.2 2.9 — — −0.5 59.3 3.00 1.9 2.4 × 10⁻² — — — — 13.6 1.8 — — −0.4 48.20.8 7.0 2.7 × 10⁻² — — — — 23.2 0.5 — — −0.3 22.7 0.2 18.9 3.1 × 10⁻² —— — — 48.1 0.3 — — −0.2 9.7 3.1 × 10⁻² 40.7 3.1 × 10⁻² — — — — 21.8 0.1— —

For comparison, the EC-CO₂RR performance was tested of commerciallyavailable, ˜50 nm diameter CuO nanoparticles (NPs) and bulk CuO powder(˜1-5 μm). The morphology, crystal structure and oxidation state ofthese oxide materials were determined by SEM, SXRD and XAS measurements(FIG. 15 ). The spectra of FIG. 15 show high crystallinity of monocliniccuprous oxide of both CuO NPs (˜50 nm diameter) and bulk CuO (˜1-5 μm).Small portion of cubic Cu₂O was detected in bulk CuO powder. Thepotential-dependent Faradaic efficiencies for all products in FIGS.16-17 show that these traditional CuO catalysts produced mostly H₂ (FE˜50-70%) with much smaller amounts of CO (FE<20%) at moderate negativepotentials and ˜20% FE of ethylene at high overpotentials. The productdistribution obtained over these catalysts is similar to those of manycopper catalysts reported before. As shown in FIGS. 6C-6D and FIG. 18 ,the CuO—IO demonstrated substantially higher FEs and selectivitiestowards CO, and CO partial current density (j_(CO)) compared with theCuO NPs and bulk CuO catalysts. The 141 mV dec⁻¹ Tafel slope for COproduction at CuO—IO was close to the 120 mV dec⁻¹ expected for a ratedetermining step involving the initial electron transfer to CO₂ (FIG. 19). Tafel slopes for the CuO NPs and bulk CuO were 178 and 184 mV dec⁻¹,respectively.

The Pourbaix diagram for the Cu—H₂O system indicates CuO should reduceto metallic Cu under EC-CO₂RR at potentials more negative than −0.5 V,which is consistent with the cyclic voltammogram (CV) of CuO—IO in CO₂saturated 0.1 M KHCO₃ (FIG. 20A). The oxidation state of Cu-basedcatalysts during CO₂RR is still debated in the literature, and in situXAS, Raman spectroscopy, and XRD experiments was conducted to monitorthe oxidation state, surface structure, crystallographic orientation,and crystallite size of the CuO—IO catalyst under electrochemicalpotential control. Cu K-edge XANES and EXAFS were collected at variouspotentials in CO₂ saturated 0.1 M KHCO₃. The EXAFS spectra in FIG. 20Bshow the CuO—IO was in the Cu²⁺ oxidation state under open circuit. Areduction of the Cu—O and Cu—Cu scattering peaks of CuO at 1.53 Å and2.52 Å, and the emergence of the first Cu—Cu coordination shell inmetallic Cu at 2.21 Å indicate the onset of Cu-oxide reduction at anapplied potential of −0.2V vs. RHE. These changes became more apparentwith increasingly cathodic potentials, and comparison with the bulk Cufoil reference indicates near complete reduction beyond −0.6V vs. RHE.These potential-dependent spectroscopic changes are consistent with theCu-oxide reduction peak centered at approximately −0.45 V vs. RHE inFIG. 20A, and they agree with the Cu—H₂O Pourbaix diagram. Theassociated Cu K-edge XANES spectra collected at different potentials andwhile being held at −0.6V are presented in FIGS. 21-22 .

In situ Raman spectroscopy was employed as a more surface sensitivetechnique to probe the surface structure changes of CuO—IO during theapplication of electrochemical potentials. Ex situ Raman spectrum inFIG. 23 shows three indicative Ag, Big, and Beg modes for fresh CuO—IOelectrode. FIG. 20C shows that the predominant Ag feature of CuO at 294cm⁻¹ gradually disappeared during the application of at −0.6 V vs. RHEin CO₂-purged KHCO₃, and no other peaks associated with Cu₂O were foundduring the reaction in the region of 100-250 cm⁻¹ and 330-600 cm⁻¹.Similar results were obtained at −0.8V and −1.0 V vs. RHE (FIG. 24 ).Metallic copper is Raman-inactive, and the immediate decrease inintensity and subsequent disappearance under electrocatalytic potentialsstrongly indicate the reduction of CuO into Cu⁰ on the electrocatalystsurface. Raman spectrum collected once the catalyst returned to opencircuit showed the presence of mixed Cu₂O and CuO species (FIG. 25 ),which reflects the reversibility of the Cu redox process

In situ SXRD collected under open circuit (FIG. 20D) identified CuObefore reaction, which is consistent with both Cu K-edge XAS and Ramanmeasurements. Under steady state operating conditions at −0.6 V vs. RHE,the presence of CuO (111), (200), (220), (311), and (222) peaksindicative of face-centered cubic Cu (space group Fm-3m) was found. Theresults identify metallic copper with lattice constant a=b=c=3.6102 Åand mean crystallite size of 10-11 nm during CO₂RR at −0.6V vs. RHE,which is consistent with pre-reaction TEM analysis. Only metallic Cu⁰was identified, and no obvious signatures were observed associated withcopper oxides under working conditions (FIG. 26A). The peak intensityratio of Cu(111) to Cu(200) for an ideal polycrystalline Cu surface wasreported to be ca. 3.03. The average relative intensity ratio of Cu(111)and Cu(200) peaks at during EC-CO₂RR at −0.6V vs. RHE is 3.58 (FIG.26B), implying the dominance of closed-packed Cu(111) surface orpreferential (111) orientation of the catalyst during the reaction. Thecrystallographic orientation and crystallite size of operating catalystdid not substantially change during five hours of measurement at −0.6Vvs. RHE. Similar to Raman measurements, re-oxidation of the catalyst wasobserved once it was returned to open circuit after the application ofcathodic potential (FIG. 27 ). Correlating these various in situmeasurements indicates that Cu⁰ with a preferred Cu (111) orientation isa dominant species present in the CuO—IO catalyst during EC-CO₂RR.

The CuO—IO catalyst demonstrated some of the highest CO₂-to-COselectivity reported for oxide-derived copper electrocatalysts to date(Table 2) at low to moderate overpotentials.

TABLE 2 Comparison of CO₂ conversion to C₁ products of present study andprevious oxide-derived Cu and Cu based catalyst reports. Major C₁ FE/Potential/V Sample product % vs. RHE References CuO-derived Cu inverseopal CO 72.5 −0.6 This work HCOOH 48.1 −0.3 Cu₂O-derived Cu CO 45 −0.35Li et al., J. Am. Chem. Soc. 2012, HCOOH 38 −0.55 134, 7231-7234CuO-derived Cu nanowire CO 50 −0.6 Ma et al., Phys. Chem. Chem. HCOOH 40−0.7 Phys. 2015, 17, 20861-20867 Cu₂O-derived Cu inverse opal CO 45.3−0.6 Zheng et al., Nano Energy 2018, HCOOH 34.5 −0.8 48,93-100Electrochemically reduced CuO- CO 62 −0.4 Cao et al., ACS Catal. 2017,7, derived Cu nanowire HCOOH 25 −0.5 8578-8587 Electrochemically reducedCuO- CO 61.8 −0.4 Raciti et al., Nano Lett. 2015, 15, derived Cunanowire HCOOH 30.7 −0.6 6829-6835 3 D CuO-derived Cu hierarchical CO 60−0.55 Raciti et al., ACS Appl. Energy nanostructures HCOOH 30~40−0.55~−0.7 Mater. 2018, 1, 2392-2398 Chrysanthemum-like Cu HCOOH 50~70−1.6 Xie et al., Electrochimica Acta nanoflower 2014, 139 137-144 Cunanoparticles/N-doped carbon CO 23 −0.7 Song et al., ChemistrySelectnanospike CH₄ 35 −0.9 2016, 1, 6055-6061 Cu nanoparticles/reduced CO 40−0.6 Hossain et al., Sci. Rep. 2017, 7, graphene oxide HCOOH 46.2 −0.43184 Cu nanoparticles/reduced CO 50 −0.4 Cao et al., J. CO₂ Util. 2017,22, graphene oxide 231-237 Cu/carbon aerogels CO 35 −0.81 Han et al.,Electrochim. Acta 2019, 297, 545-552 Cu nanoparticles/carbon aerogels CO75.6 −0.6 Xiao et al., J. Colloid Interface Sci. 2019, 545, 1-7

Selective EC-CO₂RR performance and strong HER inhibition demonstrated bythe CuO—IO electrocatalyst compositions may be attributed to both its 3Dmorphology and crystallographic surface orientation. The CuO—IO catalystis composed of small nanoparticles in a 3D interconnected porousstructure that offers a large surface-to-volume ratio. As shown in FIG.28 and Table 3, the ECSA (˜2.96 cm²) and RF (˜30.8) of CuO—IO wereconsiderably larger than the CuO NPs and bulk CuO powder (0.37˜0.55 cm²and RF=5.3˜7.8). The preferential Cu(111) surface/orientation is alsoexpected to demonstrate higher C₁ selectivity due to weaker binding of*CO and *COOH intermediates, whereas Cu(100) facets have favored C₂₊production owing to a lower energetic barrier for intermediatehydrogenation. In the low and moderate overpotential range (−0.2 to−0.8V vs. RHE), the CuO—IO catalyst produced exclusive C₁ products andalmost no H₂ evolution. In this potential range, the high surfaceroughness allowed rapid consumption of both CO₂ and H⁺ at the catalystsurface. The observed current density likely increased the local pHsufficiently to reduce the number of protons available for HER, whilethe Cu (111) orientation favored C₁ production over C₂ formation. In thehigh overpotential range, large current densities can increase the localpH at copper electrodes to 13 or higher. These basic conditions canactivate additional C₂ forming reaction pathways and deplete theconcentration of available CO₂ molecules. The effect of these twophenomena was observed beyond −0.8V vs. RHE with increased H₂ evolutionand the emergence of C₂ product formation at the CuO—IO catalyst. Whilehydrocarbon formation was observed at large overpotentials, methane wassubstantially favored over ethylene. These results were largelyconsistent with the expected C₁ preference of Cu(111) facets.

TABLE 3 ECSA and RF of CuO—IO, bulk CuO and CuO NPs electrodes. SampleECSA/cm² RF CuO—IO 2.962 30.79 Bulk CuO 0.372 5.27 CuO nps 0.548 7.76

In summary, the roughened, porous hierarchical CuO-derived IO catalysthas shown impressive CO selectivity across a wide potential range withnegligible H₂ evolution compared with bulk oxidized copper surfaces. Theelectrocatalyst compositions comprising a 3D interconnected porousstructure made of small nanoparticles in Cu-promoted EC-CO₂RR bycreating local pH gradients within the catalyst pores that deplete thelocal concentration of protons available for HER. In addition, the highroughness surface and the dominance of active Cu (111) surface sitewould facilitate a C₁ reaction path.

Having described the basic concept of the embodiments, it will beapparent to those skilled in the art that the foregoing detaileddisclosure is intended to be presented by way of example. Accordingly,these terms should be interpreted as indicating that insubstantial orinconsequential modifications or alterations and various improvements ofthe subject matter described and claimed are considered to be within thescope of the spirited embodiments as recited in the appended claims.Additionally, the recited order of the elements or sequences, or the useof numbers, letters or other designations therefor, is not intended tolimit the claimed processes to any order except as may be specified. Allranges disclosed herein also encompass any and all possible sub-rangesand combinations of sub-ranges thereof. Any listed range is easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as up to, at least, greater than, less than, and the like refer toranges which are subsequently broken down into sub-ranges as discussedabove. As utilized herein, the terms “about,” “substantially,” and othersimilar terms are intended to have a broad meaning in conjunction withthe common and accepted usage by those having ordinary skill in the artto which the subject matter of this disclosure pertains. As utilizedherein, the term “approximately equal to” shall carry the meaning ofbeing within 15, 10, 5, 4, 3, 2, or 1 percent of the subjectmeasurement, item, unit, or concentration, with preference given to thepercent variance. It should be understood by those of skill in the artwho review this disclosure that these terms are intended to allow adescription of certain features described and claimed withoutrestricting the scope of these features to the exact numerical rangesprovided. Accordingly, the embodiments are limited only by the followingclaims and equivalents thereto. All publications and patent documentscited in this application are incorporated by reference in theirentirety for all purposes to the same extent as if each individualpublication or patent document were so individually denoted.

What is claimed is:
 1. A method for the synthesis of an electrocatalystcomposition, the method comprising: providing a polymethylmethacrylatelatex; infiltrating the polymethylmethacrylate latex with a copperprecursor; and, annealing the infiltrated polymethylmethacrylate latexto provide an electrocatalyst having an inverse opal structure.
 2. Amethod for electrochemical conversion of CO₂ to CO comprising: providinga working electrode comprising an electrocatalyst, wherein theelectrocatalyst comprises a 3D interconnected porous copper inverse-opalstructure; applying a negative potential to the working electrode;contacting the working electrode with CO₂, wherein the CO₂ is reduced toCO.
 3. The method of claim 2 wherein the electrocatalyst has an averagecavity size ranging from about 175 to about 185 nm.
 4. The method ofclaim 2 wherein the inverse-opal structure has a hexagonal structure. 5.The method of claim 2 wherein the inverse-opal structure is a negativereplica of poly (methyl methacrylate) opal.
 6. The method of claim 2wherein the 3D interconnected porous copper inverse-opal structurecomprises nanoparticles with an average mean diameter ranging from about15 to about 20 nm.
 7. The method of claim 2 wherein the method has aFaradaic efficiency greater than about 65% at −0.7 V vs. RHE.
 8. Themethod of claim 7 wherein the Faradaic efficiency is greater than about70% at −0.6 V vs. RHE.
 9. The method of claim 2 wherein the CO₂ isconverted to CO with a CO to H₂ selectivity greater than about 80% at−0.8 V vs. RHE.
 10. The method of claim 9 wherein the CO to H₂selectivity is greater than about 90% at −0.7 V vs. RHE.
 11. The methodof claim 2, wherein the inverse-opal structure has a hexagonalstructure; wherein the inverse-opal structure is a negative replica ofpoly (methyl methacrylate) opal; wherein the electrocatalyst has anaverage cavity size ranging from about 175 to about 185 nm; wherein themethod has a Faradaic efficiency greater than about 70% at −0.6 V vs.RHE; and wherein the method the CO₂ is converted to CO with a CO to H₂selectivity up to about 90% at −0.7 V vs. RHE.
 12. The method of claim 2wherein the electrocatalyst comprises copper in a +2 oxidation state inthe providing step.
 13. The method of claim 2 wherein theelectrocatalyst comprises copper in a 0 oxidation state when thenegative potential is applied to the working electrode.